1. Field of the Invention
The present invention relates to an apparatus for coating a metal strip with a coating material comprising a high-viscosity liquid, such as non-solvent paints, inks and emulsions, thinnly and smoothly at a relatively high speed.
2. Description of the Prior Art
Prior to now, pick-up roll, multiple-step kneading roll, and photogravure roll methods have been used for applying a thin coating on a metal strip. These methods are selectively used depending on the type and nature of the coating materials, the coating speed and other factors.
The pick-up roll method, as shown in FIG. 1, picks up the coating material 1' by means of the pick-up roll 9 immersed in the paint pan 1, controlling the amount of the picked up coating material with the contact pressure between the coating roll 5 (or a bracket roll) and the pick-up roll, and applying the controlled amount of coating material onto the metal strip 7. A modified pick-up roll method may include controlling the amount of the picked up coating material by the distance between the pick-up roll 9 and the metering roll 10 provided in contact with the pick-up roll 9, transferring the controlled amount of the coating material onto the coating roll, and applying the coating material onto the metal strip 7.
This pick-up roll method has the advantage that the coating material can be easily applied onto the metal strip with relatively simple equipment, but has the following technical disadvantages and has very limited use in high-speed applications of a coating material composed of a high-viscosity liquid onto a metal strip.
In general, the high-viscosity liquid can not be picked-up by the roll when the roll rotates at a high speed. The experiments conducted by the present inventors have revealed that there exists a correlation as illustrated in FIG. 5 between the viscosity of coating materials and the speed of the pick-up roll. Thus, the pick-up of the coating material is impossible within the oblique-lined zone in FIG. 5.
Therefore, in cases of high-speed coating with a coating material having a viscosity not lower than 500 cp with a coating speed not lower than 50 m/min., to which the present invention is particularly directed, the coating material can not be picked-up by the pick-up roll and thus it is impossible by the pick-up roll method to apply a coating material composed of a high-viscosity liquid onto a metal strip.
The pick-up roll method has a further disadvantage in that the thickness of the resultant coatings vary depending on changes in the coating speed. For example, as clearly understood from the results shown in FIG. 6 obtained by measuring the correlation between the coating speed and the thickness of the coatings, the thickness increases in proportion to the increase of the coating speed. Therefore, in a continuous coating operation of a steel strip, where the running speed of the strip changes every time the strip is jointed to a subsequent steel strip, it becomes very difficult to obtain a uniform coating thickness on the final product.
Further, the coating material composed of a high-viscosity liquid results in further problems in that when it is used for a long-time in a high-speed coating operation, it mixes with the air and generates fine air bubbles therein as a result of being stirred by the rotation of the pick-up roll immersed therein. The bubbles generated in the coating material are transferred together with the coating material to the pick-up roll, then to the bracket roll, further to the coating roll and lastly to the metal strip where they result in coating defects, such as pin holes, and damages the coating finish. It should be noted that the bubbles entangled in the coating material are harder to remove as the viscosity of the coating material increases in addition to there being an increased likelihood of coating defects.
The second conventional method, the multiple-step kneading roll method has been used for coating of printing inks in general. As shown in FIG. 2, this method comprises leading out the ink contained in the ink pan 101 by means of the leading out roll 109, controlling the amount of the ink to be applied through the number of contacts of the transfer roll 111 with the leading out roll 109 and one of the kneading roll group 112, which is made up of a plurality of metal rolls and rubber rolls transferring the ink to others of the kneading roll group whereby the ink is spread thinnly and uniformly on the rolls and transferred to the printing roll 113. The plate cylinder has a relief or flat plate for printing.
The ink transferred onto the printing roll 113 is further transferred onto the coating roll 105 and then onto the metal strip 107. The multiple-step kneading roll method has the advantages that a high-viscosity coating material can be applied thinnly and uniformly on a metal strip running at a high speed, but has the following disadvantages.
In the multiple-step kneading roll method, the transfer of the coating materials to and among a number of rolls is largely limited by the physical properties, particularly rheological properties of the coating material itself, such as viscosity and thixotropy, and when these properties are not appropriate, it becomes impossible to spread the coating material thinnly and uniformly by means of the kneading rolls or to transfer it from roll to roll.
Therefore, in order to obtain rheological properties of the coating material suitable for coating through the multiple-step kneading roll method, it becomes necessary to adjust the coating material by mixing additional chemicals therein in a complicated manner. Also the coating material in this method is limited with respect to flowage, for example, a slurry of coating material composed of inorganic substances and water soluble resins which are used for forming a heat-resistant and insulating coating on metal articles can not be used in this method.
Further, the multiple-step kneading method requires large scale precision machines which require not only a great amount of capital and maintenance expenses, but also technical skill in controlling the thickness of the coatings.
The third conventional method, the photogravure roll method, as shown in FIG. 3, comprises picking up the paint contained in the pan 201 by means of the photogravure roll 203 immersed in the paint, removing the excess of the paint picked up by means of the doctor knife 202, transferring the paint onto the coating roll 205 and then onto the metal strip. This method has the advantages that it is easy to control the thickness of coatings because the amount of coating material is controlled by the photogravure roll and the doctor knife, but this method has the following disadvantages.
For the same reason as in the pick-up roll method, it is impossible to apply a coating material composed of high-viscosity liquids because fine bubbles are generated and entangled in the coating material and tend to cover recesses or grooves of the photogravure roll, thus hindering filling up of the recesses or grooves with the coating material. This incomplete filling of the recesses or grooves with the coating material causes serious coating defects, such as pin holes and a deteriorated flatness and smoothness of the coated surface of the metal strip.